FIG. 1 shows a typical switch-mode power supply circuit. After being subject to full-wave rectification (of a rectification bridge consisting of diodes D11, D12, D13, and D14) and a π-type filtering circuit consisting of capacitors C12 and C13 and an inductor L11, an alternating current is then converted into a high-voltage direct current, in which D13 and C11 at a secondary side form a secondary rectification filtering network that rectifies and filters a voltage at a secondary side of a transformer. A starting circuit that is formed by R13 and C14 and that is derived from the π-type filtering circuit provides a starting voltage to a control chip. Resistors R14 and R15 form a feedback voltage dividing network, and a sampling circuit in the control chip samples an output voltage coupled to a feedback winding Lk by using the feedback voltage dividing network. After sampling the output voltage, the control chip generates a control signal by means of internal processing to control a switching transistor T1 to turn on/off, thereby adjusting the magnitude of input power, so as to adapt to a change in an output load. The control chip performs current-limiting on a primary side inductor of the transformer via the current sensing resistor R12, to prevent a current flowing through the switching transistor T1 from being excessively large. However, in a production process of the switch-mode power supply, the current sensing resistor R12 is often short-circuited due to some reasons such as undesired production and device failure. After the R12 is short-circuited, current-limiting cannot be performed on the primary side inductor of the transformer. When the switching transistor T1 is switched on, the current continuously rises until the switching transistor T1 is damaged because of excessively high power or even the switch-mode power supply is broken down, thus causing a potential safety hazard during test and use.
Currently, in general practices of switch-mode power supply manufacturers, these defective products are screened out in a test process, so as to prevent these defective products from being delivered to users. Alternatively, a turn-on time of the primary side inductor is limited. That is, after the switching transistor T1 is switched on, a fixed delay time Ton is waited for, and if the voltage on the current sensing resistor R12 does not reach a maximum value in this time, it is judged that the current sensing resistor R12 is short-circuited, and the switching transistor T1 is switched off, thereby switching off the control chip.